1. Field of the Invention
The present invention relates to a driving circuit for driving a semiconductor device (semiconductor element) used in a power converter such as an inverter, which functions to protect the semiconductor device against an overcurrent condition.
2. Description of the Background Art
A driving circuit for driving an IGBT which is one example of a high power semiconductor device often has an “overcurrent protecting function”, i.e., functions to protect the IGBT against an overcurrent condition. The overcurrent protecting function is performed as follows. First, an IGBT cell is divided into a main terminal and a sense terminal. A sense current obtained from the sense terminal is converted into a voltage by a current detecting resistor, as a sense voltage. The sense voltage is monitored, thereby to detect an overcurrent condition in which a main current excessively flows through the main terminal of the IGBT. Then, upon detection of the overcurrent condition based on the sense current, the driving circuit interrupts current flow to the IGBT.
The above-described method for detecting an overcurrent condition based on the sense current (sense voltage), however, has a disadvantage. According to the above-described method, the sense voltage and the main current are in inappropriate proportion to each other during a period immediately after a turn-on or a turn-off of the IGBT, which makes it impossible to accurately detect an overcurrent condition in which the main current excessively flows.
Such inappropriate proportion between the sense voltage and the main current occurs due to imbalance between the main terminal and the sense terminal of the IGBT with respect to relationship between a collector-emitter voltage VCE and a gate-emitter voltage VGE. Such imbalance is observed during a period immediately after a turn-off or a turn-on of the IGBT, and the degree of the imbalance is particularly great during a “Miller period” (a period of discharge/charge of a feedback capacitor between a collector and a gate). This disadvantage is recognized by simulation carried out using an equivalent circuit of the driving circuit for the IGBT.
To overcome the foregoing disadvantage, a conventional overcurrent protection circuit is provided with a filter or a delay circuit having a sufficiently large time constant, to function to shape a waveform (including a delay) so as to prevent an overcurrent condition from being erroneously detected during a predetermined period immediately after a turn-off or a turn-on.
Another conventional overcurrent protection circuit functioning to shape such a waveform is disclosed in Japanese Patent Application Laid-Open No. 2001-345688, for example. An overcurrent protection circuit disclosed in this reference includes a delay circuit for generating a delay time in giving a result of comparison of a sense voltage with a reference voltage, thereby to prevent current flow to an IGBT from being erroneously interrupted due to variation in a sense current which is caused by a noise, to avoid occurrence of malfunction.
However, the conventional overcurrent protection circuits which function to shape an appropriate waveform as described above require an external component (R, C or the like) for forming a waveform-shaping filter having a sufficiently large time constant, or the like. This makes integration difficult.
Also an RTC (Real Time Control) circuit (a circuit for interrupting current flow to an IGBT at an instant when a short-circuit current flows into the IGBT) is further required in order to compensate for operational delay which is caused by delaying a sense voltage (sense current). This makes integration more difficult.